Changing 'actual' bar length doesn't work as expected.
1) Right-click on a bar and open 'Bar Properties'
2) Change the 'Actual Bar Length' value.
If the bar length is increased, the bar will take up more space on the screen, but the desired note value will not be added.
From this state, reducing the actual bar length seems to remove that value not from the actual bar length but from the nominal bar length, destroying any music written up to that point in the bar.
Musecore 4 on MacOS 12.5.1